The Russian intelligence agency FSB says it has arrested a Ukrainian journalist suspected of spying on Russia, a move likely to increase tensions between Kiev and Moscow, which have been in conflict since 2014.

The FSB claimed in a statement on Monday that Roman Sushchenko is an officer with the Ukrainian military intelligence who has been collecting information about the Russian Armed Forces.

"Roman Sushchenko purposefully collected state secret information on the activities of the Russian Armed forces and the National Guard troops, which could have harmed state defence capabilities if leaked abroad," the FSB said in a statement.

Sushchenko's employer, the Ukrainian news agency Ukrinform, condemned the arrest, saying it was carried out in breach of international law.

"Ukrinform is indignant at the illegal arrest of own correspondent ... He was detained in Moscow on the day of arrival and the Russian side, in violation of all international rules, didn't inform anybody on his detention - neither his wife nor diplomatic institutions, or Ukrinform," Ukrinform said.

It also said that the reporter has been working for them since 2002 and was based in France since 2010. Sushchenko was in Moscow last week on a private visit during his vacation and disappeared on Friday before surfacing in a Moscow jail.

Ukrinform and the detainee's wife Anzhela Sushchenko found out about Roman's alleged involvement in espionage from Russian media reports, it added.